Hard disk storage devices are used in many computer system operations. In fact, most computing systems are not operational without some type of hard disk drive or similar storage device to store the most basic computing information such as the boot operation, the operating system, applications, and the like.
The basic hard disk drive model includes a storage disk or hard disk that spins at a designed rotational speed. An actuator arm is utilized to reach out over the surface of the disk. The arm carries a head assembly that has a magnetic read/write transducer or head for reading/writing information to or from a location on the disk. The transducer is attached to a slider, such as an air-bearing slider, which is supported adjacent to the data surface of the disk by a cushion of air generated by the rotating disk. The transducer can also be attached to a contact-recording type slider. In either case, the slider is connected to the actuator arm by means of a suspension. The complete head assembly, e.g., the suspension and head, is called a head gimbal assembly (HGA).
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks evenly spaced at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Over the years, the disk and the head have undergone great reductions in their size. For example, the original hard disk drive had a disk diameter of 24 inches. Modern hard disk drives are much smaller and include disk diameters of less than 2.5 inches (micro drives are significantly smaller than that).
This continual reduction in size has placed steadily increasing demands on the technology used in the HGA, particularly in terms of power consumption, shock performance, and disk real estate utilization. One recent advance in technology has been the development of the Femto slider, which is roughly one-third of the size and mass of the older Pico slider, which it replaces; over the past 23 years, slider size has been reduced by a factor of five, and mass by a factor of nearly 100.
Some of the recent improvement has resulted from reduction in the size of the read head. FIG. 1 shows a cross section of a common read head 100. Read head 100 typically comprises a sensor 110, shields 121 and 122, and conductive leads 131-132. For the purposes of illustration, thickness will refer to horizontal dimensions in FIG. 1, height will refer to vertical dimensions in the figure, and width will refer to dimensions going into the page. In today's read heads, sensors are commonly on the order of 300 Å thick, whereas shields are typically 1-2 microns thick and 50-100 microns wide. Thus, shields 121 and 122 are significantly larger than sensor 110. Furthermore, the separation between the head 100 and disk 140 is typically only around 1-2 nm.
During head operation, the head is excited with high current, causing the temperature in the head to rise. This rise in temperature can cause shields 121 and 122 to thermally expand. Thus, shields 121 and 122, which are commonly made of NiFe, will actually protrude, causing sensor 110 to become recessed. In come cases, shields have been known to protrude as much as 1 nm. This is quite significant considering the small distance between the air bearing surface 150 and the disk 140. Protrusion of the shields hinders the reliability of the sensor interface as well as the sensitivity of the sensor.
Thus, it is desirable to restrict shield thickness to less than 100 nm. However, reducing the size of shields can adversely affect their ability perform their two primary functions: providing resolution by shielding the sensor from fields of bits other than the bit directly below; and providing contacts for external an current supply. First, when the shield material becomes thin, it can easily saturate and will no longer act as a shield. Second, the shield material does not have a high degree of conductivity, so making a shield thinner will only increase its resistivity. Furthermore, decreasing the size of a shield can cause it to become magnetically unstable.